A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Increasingly, lithographic projection tools are becoming capable of imaging a pattern on a layer at relatively high numerical aperture NA (NA>1).
Due to the nature of the imaging process, a control of the polarisation of the illumination beam becomes therefore more important. As known to persons skilled in the art, a lithographic imaging process involves an exposure of a mask pattern on a radiation-sensitive layer on a substrate. Typically, at the mask diffraction patterns are formed by interaction of the illumination beam impinging on the mask with the mask pattern. The diffraction patterns each pass through a projection system and are then imaged as the interference pattern of the diffraction patterns on the substrate layer. At higher numerical aperture, the interference of the diffraction patterns is affected by the polarisation of light in each of the light beams that build up the respective diffraction patterns. Maximal interference is achieved when the polarisation vectors of the individual interfering beams are fully parallel. If the orientation of the polarisation vectors of the beams is not parallel interference will be less, which results in a reduced contrast of the image to be formed. It is also known that polarisation state of the diffracted beams may be affected by the mask pattern, in particular as a function of the pitch of the mask pattern, and by the projection system.